The invention relates to a method for isothermal brazing of cracks or gaps in single crystal components according to the preamble of the independent claim.
The wide use of single crystal (SX) and directionally solidified (DS) components allows an increased turbine inlet temperature and therefore an increased turbine efficiency as well. Alloys, specially designed for SX/DS casting, were developed in order to make maximum use of material strength and temperature capability. During operation of such components under high temperature conditions, various types of damages can occur. For example, cracks can result from thermal cycling and foreign object impact. In addition, cracks and inclusions may be incurred during manufacture. Because the cost of the components formed from high temperature nickel base superalloys is relatively high, it is usually more desirable to repair these components than to replace them.
The following state of the art methods for repairing high temperature superalloys are generally known:
U.S. Pat. No. 5,732,467 discloses a method of repairing cracks on the outermost surface of an article having a directionally oriented microstructure and a superalloy composition. The repairing is done by coating the cleaned crack surface with a material featuring the same material composition as said article. Thereby the coated crack surface is subjected to an elevated temperature and isostatic pressure over a period of time sufficient to repair the crack surface without changing the crystalline microstructure of the parent article.
In addition, a number of alternative methods of brazing for repairing cracks or gaps are known. U.S. Pat. No. 5,666,643 discloses a braze material for repairing an article, in particular components made from a cobalt and a nickel-base superalloy, such as gas turbine engine parts. The braze material is composed of particles featuring a high melting temperature which are distributed within the braze alloy. These particles could be of single crystal, directionally solidified, or equiaxed microstructure. But, even if particles featuring a single crystal structure are used, the structure of the repaired crack as a whole due to the braze alloy differs with respect to material properties from the single-crystal structure of the base material which leads to weakness problems of the brazed joint. This is especially valid for cracks located at stress concentrations.
The same problem occurs with the repair methods disclosed in U.S. Pat. No. 4,381,944 or U.S. Pat. No. 5,437,737 where a braze alloy and a filler material are used at the same time to increase the strength of the brazed joint. Another method of repairing by sintering is disclosed in U.S. Pat. No. 5,156,321.
It is object to the present invention to find an advanced process of joining or repairing cracks or gaps in a single crystal article made from a Nickel based superalloy by means of isothermal, epitaxial single crystal solidification of a brazing alloy.
This objective is addressed by a process wherein the Temperature of the isothermal solidification is between TLiquids Braze +5*(wt-%BBraze) and (Tsolidus, base material xe2x88x9270*(wt-%BBraze)), while (wt-%BBraze*wt-%CrBraze) is between 15 and 40 and (Tsolv.xcex3, base materialxe2x88x92TLiqidus, Braze) is above 140xc2x0 C.
These conditions lead to a homogeneous xcex3/xcex3xe2x80x2-microstructure with mechanical properties of the brazed joint similar to those of the parent material.
Stringent performance requirements dictate regular overhaul schedules which makes brazing of single crystal components an imperative and economical process.
The single crystal brazing conditions will fully maintain the single crystal structure in the braze-repaired crack resulting in a Young""s Modulus as low as that of the base material. This leads to a high Thermal Fatigue (TF) resistance and Thermal Mechanical Fatigue (TMF) resistance as well to a high Low Cycle Fatigue (LCF) value in the braze-repaired areas.
With advantage, the braze material is Ni based and contains (wt-%) 8-15 Cr and (wt-%) 1-3 B.
The heat treatment of the isothermal solidification takes place at a temperature of 1120-1160xc2x0 C. for 8 to 20 h, preferably at a temperature of 1140xc2x0 C. For reasons of in-situ adjusting of the microstructure of the brazed joint there can be a heat treatment of 1180xc2x0 C. for 30 min after the heat treatment. In addition, to allow a good melting of the brazing material there will be, before the heat treatment of 1120-1160xc2x0 C., a heat treatment of 1180 to 1200xc2x0 C. for 20 to 30 min followed by a cooling of 1-2xc2x0 C./min.
After brazing the temperature is lowered at a rate of 1 -2xc2x0 C./min to a temperature between 800-900xc2x0 C. and it is held for 1 to 6 hours to precipitate xcex3xe2x80x2.
The crack or gap will have a maximum width of 300 xcexcm. The braze slurry will be applied into and over the crack or gap and a mixture of brazing alloy and filler material is applied on top of the braze slurry before applying the heat treatment of the brazing. A protective coating can be removed before applying the method and the protective coating is reapplied after applying the method. The surface of the crack or gap can be cleaned from oxides before applying the method. A Flour-Ion-Cleaning-Method can be used for cleaning the surface before applying the process.